Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing [AlO4/2]− and SiO4/2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents (SDAs) such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces of the zeolite as well as on internal surfaces within the pores of the zeolite.
In 1982, Wilson et al. developed aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of the same properties of zeolites, but are silica free, composed of [AlO4/2]− and [PO4/2]+ tetrahedra (See U.S. Pat. No. 4,319,440). Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for [PO4/2]+ tetrahedra to produce the SAPO molecular sieves (See U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [M2+O4/2]2− tetrahedra for [AlO4/2]− tetrahedra, which yield the MeAPO molecular sieves (see U.S. Pat. No. 4,567,029). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the introduction both of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (See U.S. Pat. No. 4,973,785). Structure directing agents employed in these works were amines and organoammonium cations.
Before the SAPO materials of U.S. Pat. No. 4,440,871 were known, there were attempts to make “phosphate zeolites,” i.e., substitution of phosphorus for silicon in an aluminosilicate. Such a substitution in an aluminosilicate zeolite, [PO4/2]+ for [SiO4/2], represents a reduction of the negative charge on an aluminosilicate framework. The initial work by Flanigen and Grose co-precipitated the components of silicoaluminophosphate gels, isolated the resulting solid, suspended the resulting solids in alkali hydroxide solutions and treated them under hydrothermal conditions, yielding a series of phosphate zeolites, including those of LTL, CHA, LTA, and GIS topologies (See E. M. Flanigen and R. W. Grose, Advances in Chemistry Series No. 101, ACS, Washington D.C., 1971). The low phosphate preparations, P/Al≤1.1, resulted in alkali silicoaluminophosphate species that were not as thermally stable as their aluminosilicate analogs, often less than 350-400° C., and reduced adsorption capacity in some cases suggest the possibility of some occluded phosphate in pores and cages. Similarly, Wacks et al. disclose a process for preparing silicoaluminophosphate zeolites that entails digesting hydrated aluminophosphate solids in the presence of sodium silicate solutions to make the desired silicoaluminophosphate materials, in which the claimed range of phosphate incorporation was given by P2O5/Al2O3=0-0.2, suggesting that the Al/P≥5 in these materials (See K. Wacks et. al., U.S. Pat. No. 3,443,892). While eight examples of this zeolite synthesis process are disclosed in U.S. Pat. No. 3,443,892, there is no data offered that shows that any P was actually incorporated into the zeolite product, which is a possibility given the claimed range. Many attempts to make silicoaluminophosphate zeolites resembled reactions that would be used to make aluminosilicate zeolites, but carried out in the presence of phosphate, yielding little phosphate incorporation. Kuhl conducted syntheses of silicoaluminophosphate compositions, employing high levels of both phosphate and hydroxide, utilizing a combination of tetramethylammonium and sodium hydroxides for the latter, to make the LTA-related species ZK-21 and ZK-22 (See G. H. Kuhl, Inorganic Chemistry, 10, 1971, p. 2488). These species exhibit low phosphate incorporation, Al/P>8.9, and it was concluded that the phosphate was occluded in zeolitic cages rather than incorporated into the framework. Casci et al. disclose low phosphate chabazite materials in which the framework phosphorus is claimed to be between 0.05-5 mole %, i.e., P/(Al+Si+P)=0.0005-0.05 (See US 2014/0193327). The amount of phosphate employed in the reaction mixtures of the examples are low (Al/P>5.5) and no data is offered in the examples to show what the P incorporation actually is. An outlier disclosed in the SAPO patent (U.S. Pat. No. 4,440,871) uses some sodium aluminate, tetramethylammonium hydroxide and low phosphate (P/Al=0.4) to prepare SAPO-42 (Example 48), which has the LTA topology and a composition similar to that of ZK-21 and ZK-22 mentioned above as Al/P>10. The SAPO-42 product is described in the application by an essential formulation that does not include alkali, since U.S. Pat. No. 4,440,871 only covers compositions of the formulation mR:(SixAlyPz)O2. This patent application also discloses the synthesis of SAPO-20 from the same reaction mixture treated at higher temperature (Example 28). The SAPO-20 product, which has the SOD topology, is not porous, but has a slightly enhanced P content as Al/P=3.17.
For many years now, a large gap has been present in the known compositions of microporous silicoaluminophosphates, between the SAPOs disclosed in U.S. Pat. No. 4,440,817 and what are essentially the “phosphate zeolites” reviewed in the previous paragraph. In particular, the materials of intermediate silicon and phosphorus levels are missing, materials of intermediate charge density, of higher charge density than the SAPOs originating from low level substitution of Si into neutral AlPO frameworks. More recently, Lewis et al. addressed this gap by developing solution chemistry that led to higher charge density SAPO, MeAPO, and MeAPSO materials, enabling greater substitution of SiO4/2 and [M2+O4/2]2− into the framework for [PO4/2]+ and [AlO4/2]−, respectively, compared to U.S. Pat. No. 4,440,817, using the ethyltrimethylammonium (ETMA+) and diethyldimethylammonium (DEDMA+) SDAs. These materials include ZnAPO-57 (U.S. Pat. No. 8,871,178), ZnAPO-59 (U.S. Pat. No. 8,871,177), ZnAPO-67 (U.S. Pat. No. 8,697,927), and MeAPSO-64 (U.S. Pat. No. 8,696,886). These materials exhibited framework charge densities up to −0.15/T-atom. The relationship between the increasing product charge densities and reaction parameters, namely the ETMAOH(DEDMAOH)/H3PO4 ratios, were outlined in the literature (See Microporous and Mesoporous Materials, 189, 2014, 49-63).
Applicants have now taken another step to further address this charge density gap between low charge density SAPOs and “phosphate zeolites.” Applicants have now synthesized a new family of charged silicometallophosphate framework materials which contain both alkali metal and organoammonium cations in the pores. These materials have higher charge densities than the SAPOs of U.S. Pat. No. 4,440,871, which is enabled by the incorporation of alkali cations in addition to organoammonium cations into the synthesis and ultimately into the pores of the products. A synthesis process to make these intermediate charge density compositions includes the use of excess phosphate (8>P/Al>2), a judicious use of hydroxide, preferably 1.25≤OH−/H3PO4≤2.5, combined with an appropriate ratio of alkali and quaternary ammonium cations, all of which allow the isolation of the desired midrange P and Si compositions and avoid the “phosphate zeolites,” discussed above. Unlike the art disclosed above, the reaction mixtures from which the high charge density SAPOs disclosed here are prepared are often clear solutions. The high charge density SAPO materials of this invention are crystalline microporous compositions and they are thermally stable to at least 450° C. and often contain “Si islands.”